Utilization of a 3D printer to fabricate boluses used for electron therapy of skin lesions of the eye canthi

Abstract This work describes the use of 3D printing technology to create individualized boluses for patients treated with electron beam therapy for skin lesions of the eye canthi. It aimed to demonstrate the effectiveness of 3D‐printed over manually fabricated paraffin boluses. The study involved 11 patients for whom the construction of individual boluses were required. CT scans of the fabricated 3D‐printed boluses and paraffin boluses were acquired and superimposed onto patient CT scans to compare their fitting, bolus homogeneity, and underlying dose distribution. To quantify the level of matching, multiple metrics were utilized. Matching Level Index (ML) values ranged from 0 to 100%, where 100% indicated a perfect fit between the reference bolus (planned in treatment planning system) and 3D‐printed and paraffin bolus. The average ML (± 1 SD) of the 3D‐printed boluses was 95.1 ± 2.1%, compared to 46.0 ± 10.1% for the manually fabricated paraffin bolus. Correspondingly, mean doses were closer to the prescribed doses, and dose spreads were less for the dose distributions from the 3D‐printed boluses, as compared to those for the manually fabricated paraffin boluses. It was concluded that 3D‐printing technology is a viable method for fabricating boluses for small eye lesions and provides boluses superior to our boluses manually fabricated from paraffin sheets.

radiotherapy than extensive surgery, [3][4][5] as radiotherapy can achieve excellent results in terms of local control, cosmetics, and functionality. 6,7 EBT is typically used, due to the limited extent to which the beams penetrate, leaving healthy tissues distal to the planning target volume (PTV) protected, while delivering an acceptably uniform dose distribution to the PTV.
According to the International Commission on Radiation Units and Measurements (ICRU) Report 71, the dose distribution should result in complete PTV coverage by the 90% (of assigned dose) dose surface. 8  The potential negative impact of each of the aforementioned factors on the desired dose distribution can be reduced by the use of an individualized bolus, which is a tissue-like material leveling the surface of the patient to be approximately perpendicular to central axis. Such a bolus increases skin dose, homogenizes dose heterogeneity resulting from the irregular skin surface, and protects distal tissues by conforming dose line to the distal PTV surface. 9 Similarly, bolus electron conformal therapy (ECT) has been shown clinically useful for many clinical sites 10 such as chest wall irradiation (postmastectomy irradiation), [11][12][13] head and neck radiotherapy, 14-23 paraspinal muscle region, 24 and extremities. 25 Most of the published papers describe larger PTVs using electron fields with the bolus having a shaped upstream surface; however, papers describing bolus ECT of the nose 26 are similar to the present case of eye canthi, where bolus is used primarily to remove dose heterogeneity due to the irregular surface of the nose.
A number of factors are considered when designing the bolus; most notably, the ability of the electrons to correctly penetrate to depth at which the target is located. The shape, size, and thickness (height) of the bolus is designed in the treatment planning system (TPS), based on computed tomography of the patient, as a separate structure having a density equal to the average density of the soft tissue, 1 g/cm 3 . The next step, the production of a bolus designed in the TPS, is the most difficult and the most crucial in the process.
One of the methods of bolus production involves hand molding on the patienta method vitiated by the very fact that pressing individual layers of paraffin or polymer bolus directly on the patient's skin will affect the shape of the bolus and dose distribution in relation to the reference model (planned in TPS). Another methods of bolus production is using milled electron bolus technology, being fabricated using computer-controlled milling machines. 9,13,22,24 Although milled bolus ECT technology is the only commercially available design and fabrication technology (decimal LLC, Sanford, FL, USA), its availability is largely limited to the United States of America.
Recently, there has been increased interest in investigating three-dimensional (3D) printing technologies to produce patient-specific objects for use in a medical context, including boluses in radiotherapy. [9][10][11][12][13][22][23][24][25][26][27][28][29][30][31] Although 3D printing technology offers a viable technology for small boluses designed by individual clinics, its clinical use in radiotherapy is relatively recent. One reason for this is the difficulty in obtaining a medical certificate for materials used in 3D printers, which involves obtaining the agreement of the ethics committee and consent of the patient, as contact between the printed bolus and the surface of the patient's body. Furthermore, there is a lack of worldwide commercial availability of bolus design tools, such as those reported by Low et al. 31 and Su et al., 25 which are available from .decimal LLC.

| METHODS
The study involved 11 patients treated for skin lesions located in the corner of the eye in the West Pomeranian Oncology Centre in Szczecin, Poland ( Table 1). The patients received a treatment regimen that included the administration of a therapeutic dose of 60 Gy in 30 fractions. Each PTV shape was determined and constructed by a radiation oncologist by adding a 3 mm isotropic margin to the CTV (Clinical Target Volume). Treatment was planned in the Nucletron Oncentra MasterPlan, version 4.3 TPS using a single electron beam of 6 or 9 MeV. The dose distribution was calculated using the Voxel Monte Carlo calculation algorithm 32 and at least 50,000 number of histories/cm 2 , which equals 50,000 incident electrons/cm 2 . Using computed tomography (CT) scans with 2 mm slice thickness, a bolus individual to each patient was designed in the TPS (reference bolus), which was then produced using a 3D printera 3D-printed bolus.
The reference bolus was designed as a water-equivalent structure having its distal surface conforming to the skin surface; its flat, proximal surface was drawn perpendicular to beam central axis at a location that conformed the 90% dose surface to the distal PTV surface as closely as possible. Once an acceptable dose distribution was confirmed, the bolus was produced using a 3D printer. To demonstrate the advantages of the 3D-printed bolus over the methods of manual bolus preparation, a paraffin bolus (prepared manually, directly on the patient's body) was also created for each of the 11 patients.
Printing a bolus requires the information on the shape of a bolus, which was previously designed using the TPS and stored in DICOM format, to be converted to a format able to be recognized by the 3D printer, namely the STL format. 27 Each 3D-printed bolus was pre- To build each paraffin bolus, in TPS, the reference bolus structure was divided into a series of horizontal layers corresponding to the thickness of a single paraffin sheet (1.5 mm). From the shape of layers, the corresponding fields were made, displayed, and manually drawn on the patient's skin during the radiotherapy simulation. On the basis of data obtained from the TPS paper templates were prepared. Next, based on the paper templates, the paraffin sheets were produced. Finally, paraffin bolus was created directly on the patient skin by placing and joining together layers of paraffin (according to the shape drawn on the skin). Total times to prepare paraffin boluses varied with their size, ranging from 0.5 to 2 hours.
For quality assurance purposes, a CT scan of each bolus (both 3Dprinted bolus and paraffin bolus) was acquired without the patient.
These data were used to evaluate the homogeneity of the boluses. In addition, in TPS, the CT image was downloaded. Using landmark registration option (by selecting three corresponding points on the vertices of images of boluses), the images of the 3D-printed and paraffin bolus were superimposed onto the CT image of the patient with the reference bolus. The resulting image was used to verify the fit of the 3D-printed and paraffin bolus with the reference one, and also for simulation of dose distribution when including each bolus (3D-printed and paraffin) in the treatment plan. To quantitatively measure degree to which the two boluses accurately mapped to the reference one, the authors created a Matching Level Index (ML), based on the coverage index widely used in radiotherapy: where V 1 is the volume of the 3D-printed or paraffin bolus contained in a volume of reference bolus; V r represents the total volume of the reference bolus; and V 2 is the volume of the 3D-printed or paraffin bolus (Fig. 1).
where D r is dose for the reference bolus and D m is dose for the 3Dprinted or paraffin bolus.

| RESULTS
Based on the bolus volume data from the TPS, Table 2 was created.
In the case of the 11 patients analyzed, the 3D-printed bolus was

| DISCUSSION
The use of 3D-printing technology to produce a bolus can improve the quality of the implemented treatment. Our study has shown that a 3Dprinted bolus has three main advantages over a paraffin bolus manually constructed using paraffin slabs: (1) it is a more precise reconstruction F I G . 1. Volumes used to calculate Matching Level (ML) index (Eq. 1): V r represents the total volume of the reference model; V 2 is the volume of the 3D-printed or paraffin bolus; and V 1 is the volume of the 3D-printed or paraffin bolus contained in V r .
T A B L E 2 Results of matching 3D-printed and paraffin boluses to the reference model for 11 patients.  Given results of this study and those of others, 27,29,30 we are confident that 3D-printed boluses can be used accurately and safely for electron beam therapy.
Authors obviously see the possible limitations. 3D-printing times for the small boluses used in the present study ranged from 0.5 to 5 hours. Such long printing times could restrict the size of the printed boluses, e.g., those used for chest wall electron therapy. 13,24,28,30 Of course, the cost of the 3D printer and materials should be taken into consideration. 30

| CONCLUSION
In conclusion, 3D printing technology is a viable method for fabricating boluses for small eye lesions and provides boluses superior to ours manually fabricated from paraffin sheets. Additionally, dose distributions using 3D-printed boluses more closely matched those of the dose plan than those manually fabricated.